Various vapor deposition processes are known to be useful, for example, in the production of semiconductor products. These processes are typically used to deposit very thin layers of various substances including conductive, semiconductive and insulative materials onto a substrate. Vapor deposition processes typically require each deposited material to be transported to the deposition chamber in a gas state or vapor phase where it is condensed onto the work in process.
Efficient operation of such a deposition process requires precise control of the pressure of the gases or vapors used in the process. Where the deposit material in its vapor phase has a relatively low condensation temperature (i.e., well below room temperature), the pressure of the material may be controlled using pressure transducers operating at room temperature. However, where the gas state or vapor phase of a deposit material has a relatively high condensation temperature, i.e., above room temperature, to avoid condensation, such materials are heated and maintained above their condensation temperatures, and thus heated transducers are usually required for measuring the pressures of these hot gases and vapors. Heated pressure transducers are also often heated to prevent sublimation or precipitation of solid material. For example, as is well known, ammonium chloride (NH4Cl) is a chemical by-product of processes for depositing layers of silicon nitride (Si3N4), and if the pressure and temperature drop too low the NH4Cl sublimates so that a solid salt forms on any exposed cool surfaces. To prevent such sublimation of NH4Cl, these processes are often conducted at 150° C.
FIG. 8 shows a sectional view of a portion of a prior art heated pressure transducer assembly 100 of the type that is typically used in connection with relatively high temperature, vapor deposition processes. The transducer 100 includes several major components such as an external shell 110, a heater shell 120, a heater 130, a capacitive pressure sensor 140, a front end electronics assembly 160, a heater control electronics assembly 170, and an input/output (I/O) electronics assembly 180. As will be discussed in greater detail below, the transducer 100 generates an output signal indicative of a pressure measured by sensor 140.
For convenience of illustration, many mechanical details of the transducer 100, such as the construction of the sensor 140 and the mounting of the sensor 140 and the electronics assemblies 160, 170, 180, have been omitted from FIG. 8. However, heated capacitive pressure transducers such as transducer 100 are well known and are described for example in U.S. Pat. No. 5,625,152 (Pandorf); U.S. Pat. No. 5,911,162 (Denner); and U.S. Pat. No. 6,029,525 (Grudzien).
Briefly, the external shell 110 includes a lower enclosure 112, an upper electronics enclosure 114, and a joiner 116 that holds the enclosures 112, 114 together. The heater shell 120 is disposed within the lower enclosure 112 and includes a lower enclosure or can 122 and a cover 124. The sensor 140 and the front end electronics assembly 160 are disposed within the heater shell 120, while the heater control electronics assembly 170 and the I/O electronics assembly 180 are disposed within the upper electronics enclosure 114.
The heater 130 includes a barrel heater 132 wrapped around the can 122 and an end heater 134 that is secured to a bottom of the can and is electrically connected to the barrel heater 132 via wires 136. A temperature sensor (e.g., a thermistor) 190 is fixed to an internal surface of heater shell 120.
The sensor 140 includes a metallic, flexible, diaphragm 142 and a pressure tube 144 that extends from an area proximal to the diaphragm through the heater shell 120, and through the lower sensor enclosure 112. The lower, or external, end of tube 144 is generally coupled to a source of fluid (not shown). Pressure of fluid in the source is communicated via the tube 144 to the lower surface of diaphragm 142 and the diaphragm 142 flexes up or down in response to changes in pressure within the tube 144. The diaphragm 142 and a reference conductive plate of the sensor 140 form a capacitor, and the capacitance of that capacitor varies in accordance with movement or flexion of the diaphragm. Accordingly, that capacitance is indicative of the pressure within the tube 144. The front end electronics assembly 160 and the I/O electronics assembly 180 cooperatively generate an output signal representative of the capacitance of sensor 140 which is, of course, also representative of the pressure within the tube 144. The I/O electronics assembly 180 makes that output signal available to the environment external to transducer 100 via an electronic connector 182.
FIG. 9 shows one example of how a capacitive pressure sensor 140 can be constructed. Capacitive pressure sensors of the type shown in FIG. 9 are discussed in greater detail in U.S. Pat. No. 6,029,525 (Grudzien). The sensor 140 shown in FIG. 9 includes a circular, conductive, metallic, flexible diaphragm 142, a pressure tube 144, and an electrode 246. The electrode 246 and the diaphragm 142 are mounted within a housing 248. The electrode 246 includes a ceramic block 250 and a conductive plate 252. The ceramic block 250 is rigidly mounted to the housing 248 so that a bottom face of block 250 is generally parallel to, and spaced apart from, the diaphragm. The bottom face of block 250 is normally planar and circular. The conductive plate 252 is deposited onto the bottom face of block 250 and is also generally parallel to, and spaced apart from, the diaphragm. The conductive plate 252 and the diaphragm 142 form two plates of a variable capacitor 254. The capacitance of capacitor 254 is determined in part by the gap, or spacing between, the diaphragm 142 and the conductive plate 252. Since the diaphragm flexes up and down (thereby changing the spacing between diaphragm 142 and conductive plate 252) in response to pressure changes in tube 144, the capacitance of capacitor 254 is indicative of the pressure within tube 144.
FIG. 9 shows only one of the many known ways of configuring a capacitive pressure sensor 140. However, capacitive pressure sensors 140 generally include one or more conductors that are held in spaced relation to a flexible, conductive, diaphragm. The diaphragm and the conductors form plates of one or more variable capacitors and the capacitance of those capacitors varies according to a function of the pressure in tube 144.
Returning to FIG. 8, ideally, the output signal of transducer 100 varies only according to changes in the pressure of the fluid in tube 144. However, changes in the temperature of the transducer 100, or temperature gradients within the transducer 100, can affect the output signal. This is primarily due to the different coefficients of thermal expansion of different materials used to construct the sensor 140. A secondary effect relates to the temperature sensitive performance of the front end electronics 160. Accordingly, the accuracy of transducer 100 can be adversely affected by temperature changes in the ambient environment.
To minimize the adverse effect of changing ambient temperature, the temperature sensitive components of the transducer 100 (i.e., the sensor 140 and the front end electronics 160) are disposed within the heater shell 120, and in operation the heater 130 heats the heater shell 120 to a controlled, constant temperature. The heater 130 and the heater shell 120 essentially form a temperature controlled oven that maintains the temperature of the temperature sensitive components at a constant pre-selected value.
In operation, the heater control electronics assembly 170 applies an electrical signal to the heater 130 via wires 172. The heater control electronics assembly 170 normally includes components for monitoring the temperature of the heater shell 120 via sensor 190 and adjusting the signal applied to the heater 130 so as to maintain the shell 120 at a constant temperature.
While it is necessary to heat the sensor 140 and the front end electronics 160, it is preferable to keep the heater control electronics assembly 170 and the I/O electronics assembly 180 at a relative low temperature to decrease their failure rate. Therefore, the joiner 116 that holds the lower enclosure 112 to the upper enclosure 114 is made from a thermally conductive material, such as aluminum, to dissipate heat from the heater 130 and the heater shell 120 and away from the heater control electronics assembly 170 and the I/O electronics assembly. Alternatively, or in addition, the transducer may be provided with vents and thermal shunts to enable convective cooling between the lower enclosure 112 and the upper enclosure 114. Examples of vents and thermal shunts are shown in U.S. Pat. No. 5,625,152 (Pandorf et al.). Another method of cooling the upper enclosure 114 includes physically spacing, or separating, the upper enclosure 114 from the lower enclosure 112. Active cooling can also be employed using fans or thermoelectric coolers.
What is still desired is a new pressure transducer assembly having improved thermal characteristics, wherein one housing enclosure of the transducer is maintained at a low temperature while another housing enclosure is maintained at a high temperature. Preferably, the transducer will be relatively compact, with the housing enclosures closely coupled. In addition, the transducer will preferably not require the use of active cooling devices or vents allowing direct airflow over electronics assemblies contained in the housing enclosures.